Apparatus for washing fibrous material

ABSTRACT

APPARATUS FOR WASHING A FIBROUS, CELLULOSIC MATERIAL SUSPENDED IN A LIQUID COMPRISES A DRUM HAVING A SHELL PERVIOUS TO THE SUSPENSION LIQUID AND SUBSTANTIALLY NONPERVIOUS TO THE FIBROUS MATERIAL, A MEMBER DEFINING A WEBFORMING ZONE ALONG THE PORTION OF THE DRUM, A LIQUID PERVIOUS MEMBER DEFINING A WASHING ZONE ALONG A SECOND PORTION OF THE SHELL AND A FLEXIBLE FORAMINOUS BELT TRAINED AROUND THE DRUM WITHIN THE MEMBERS AND MOUNTED FOR MOVEMENT WITH THE DUM SURFACE. THE SUSPENSION LIQUID IS INTRODUCED AT THE UPSTREAM END OF THE WEB-FORMING ZONE BETWEEN THE FORAMINOUS BELT AND THE DRUM SURFACE. THE MEMBER DEFINING THE WEB-FORMING ZONE IS FORMED AND POSITIONED SUCH THAT THE WEB-FORMING ZONE IS TAPERED IN THE DIRECTION OF ROTATION OF THE DRUM, AND THUS UNDER THE INFLUENCE OF THE PRESSURE SUPPLY OF THE SUSPENSION, THE FIBROUS MATERIAL IS DRAINED INWARDLY THROUGH THE SHELL AND A WEB IS FORMED ON THE DRUM SURFACE. WASH LIQUID IS SUPPLIED UNDER PRESSURE FROM OUTSIDE THE LIQUID PERVIOUS MEMBER DEFINING THE WASHING ZONE AND DISPLACES SUSPENSION LIQUID FROM THE WEB. THE LIQUIDS EXPRESSED AND DISPLACED FROM THE WEB ARE CONDUCTED AWAY FROM INSIDE THE DRUM. THE LIQUID PERVIOUS MEMBER DEFINING THE WASHING ZONE IS SO FORMED AND POSITIONED RELATIVE TO THE DRUM SURFACE THAT THE THICKNESS OF THE WEB IS MAINTAINED AT ALL POINTS THROUGHOUT THE WASHING ZONE AT A VALUE NOT GREATER THAN THE THICKNESS OF THE WEB AT ANY POINT IN THE WEB FORMING ZONE. CONSEQUENTLY, THE FIBROUS MATERIAL IS NOT ALLOWED TO EXPAND IN THE WASHING ZONE. THE APPARATUS ALSO COMPRISES A PRESS ROLL DOWNSTREAM OF THE WASHING ZONE FOR DEWATERING THE WEB AND A DEVICE FOR REMOVING THE WEB FROM THE APPARATUS.

Nov. 2, 1971 INGERMARSSON 3,616,660

APPARATUS FOR WASHING FIBROUS MATERIAL Filed May 22, 1969 2 Sheets-Sheet1 Fig.1

INVENTOR.

GOSTA INGEMAR INGEMARRSON Q Z X ZQ W his A T TORNEYS 1971 G. I.INGERMARSSON APPARATUS FOR WASHING FIBROUS MATERIAL Filed May 22, 1969 2Sheets-Sheet 2 Fig.2

IN VENTOR. GOSTA INGEMAR INGEMARRSON BY 9,4 Qpdfi/W his ATTORNEYS3,616,666 Patented Nov. 2., 1971 3,616,660 APPARATUS FOR WASHING FIBROUSMATERIAL Gosta Ingemar Ingermarsson, Karlstad, Sweden, assignor toAktiebolaget Karlstads Mekaniska Werkstad, Karlstad, Sweden Filed May22, 1969, Ser. No. 826,873

Claims priority, application Sweden, May 24, 1968,

7,004/ 68 Int. Cl. D06f 29/00 US. Cl. 68-22 R 15 Claims ABSTRACT OF THEDISCLOSURE Apparatus for washing a fibrous, cellulosic materialsuspended in a liquid comprises a drum having a shell pervious to thesuspension liquid and substantially nonpervious to the fibrous material,a member defining a webforming zone along the portion of the drum, aliquid pervious member defining a Washing zone along a second portion ofthe shell and a flexible foraminous belt trained around the drum withinthe members and mounted for movement with the drum surface. Thesuspension liquid is introduced at the upstream end of the web-formingzone between the foraminous belt and the drum surface. The memberdefining the web-forming zone is formed and positioned such that theweb-forming zone is tapered in the direction of rotation of the drum,and thus under the influence of the pressure supply of the suspension,the fibrous material is drained inwardly through the shell and a web isformed on the drum surface. Wash liquid is supplied under pressure fromoutside the liquid pervious member defining the washing zone anddisplaces suspension liquid from the web. The liquids expressed and displaced from the web are conducted away from inside the drum. The liquidpervious member defining the washing zone is so formed and positionedrelative to the drum surface that the thickness of the web is maintainedat all points throughout the washing zone at a value not greater thanthe thickness of the web at any point in the web forming zone.Consequently, the fibrous material is not allowed to expand in thewashing zone. The apparatus also comprises a press roll downstream ofthe washing zone for dewatering the web and a device for removing theweb from the apparatus.

BACKGROUND OF THE INVENTION This invention relates to apparatus forwashing fibrous material suspended in a liquid and is particularlyadapted for washing fibrous, cellulosic material.

Wood pulp and other materials containing cellulose are treated toliberate and yield cellulose by first cooking a suspension of thematerial in a liquid, usually water, such as in a pulp digester. Thesuspension liquid is then separated from the fibers to the greatestextent possible. Inasmuch as some of the chemicals present in the liquidcan be returned to the pulp process while others may find use as a fuel,and moreover in the interest of water conservation, the liquidsseparated from the cellulosic fibers are recovered. Usually, the processof recovery of useful substances includes an evaporation step, and it istherefore advantageous to minimize the quantities of liquids utilized inthe cellulose recovery process to the utmost extent. One aspect of therecovery of the cellulose involves washing it after removing suspensionliquid, and it is desirable to limit the amount of washing liquid.

One of the common types of apparatus used for washing fibrous cellulosematerial is the filter-type washer. The fibrous suspension from thedigester is first diluted to a suitable consistency and is introducedinto a vessel or trough. A rotatable filter drum is partially immersedin the vessel or trough, and the hydrostatic pressure difference betweenthe inside of the drum and the suspension liquid causes the liquid todrain through the drum shell and a web to be formed on the drum surface.After the web emerges from the suspension, upon further rotation of thedrum, washing liquid is sprayed through the material and is drainedthrough the drum shell. After draining, the fibrous web is removed fromthe drum surface. An amount of liquid equal to the amount of washingliquid sprayed onto the web is removed from the vessel or trough, whilethe remaining liquid is circulated back to the input to the filterwasher and used to dilute the incoming fibrous suspension.

One of the principal disadvantages of a filter-type washer is therequirement for circulating large quantities of liquid to form the webon the drum surface. The equipment required is expensive and alsorequires a large amount of space. Moreover, the apparatus is subject tofoaming problems. Another important disadvantage is that the fibrousweb, at the time that washing liquid is sprayed on it, has a relativelylow solids content, so that lumen liquids and some liquids present inthe voids between the fibers are not effectively removed by the washing.Moreover, the fibers of the fibrous web are relatively mobile, and themovement of the fibers tends to reduce the effectiveness of washing.

Another type of washing is so-called press-washing. In press-washing,the suspension of fibrous material is diluted and then introduced into apressing device, usually a screw press. The pressing of the fibrousmaterial yields a fibrous material having a high solid content, andlumen liquid generally participates in the wash process. Generally,press-washing process requires less washing liquid. However, thepress-washing process has the disadvantage of requiring several stagesof pressing, and the equipment tends to be complex and subject tomechanical difficulties.

SUMMARY OF THE INVENTION There is provided, in accordance with theinvention, a novel and improved apparatus for washing fibrous material,such as cellulosic material, that overcomes the disadvantages associatedwith presently known devices of the above types and provides importantadvantages in its own right, such as requiring a minimum of wash liquid,producing a fibrous material substantially free of any remainingsuspension liquid, and producing a fibrous material having a high drycontent. The apparatus is relatively trouble-free, and thus requires aminimum of maintenance and shut-down time.

More particularly, the apparatus, according to the invention, comprisesa rotatable drum having a shell pervious to the liquid and substantiallynon-pervious to the fibrous material to be washed, a member positionedadjacent to the drum and defining a tapered web-forming zone extendingover a portion of the drum circumference and a liquid pervious memberdefining with the drum surface a washing zone downstream from theweb-forming zone and extending over a second portion of the drumsurface. A flexible, foraminous belt, such as wire mesh belt, is trainedaround the drum within the two members and is arranged to rotate withthe drum surface. The fibrous suspension to be washed is introducedunder pressure to the upstream end of the webforming zone between thedrum surface and the foraminous belt, and as the drum surface and beltconverge through the tapered web-forming zone, suspension liquid isexpressed from the fibrous material into the drum, thus resulting in theformation of a web of fibrous material between the drum shell and theforaminous belt. Preferably, the member defining the web-forming zone ispervious to liquid, and the fibrous material drains outwardly throughthe belt and members. Liquid expressed outwardly may be collected in acollector tray or chamher.

The liquid-pervious member defining the washing zone is maintained inpredetermined spaced relation to the drum surface and is formed,preferably with a curvature matching that of the drum shell, such thatthe thickness of the web as it traverses the washing zone is maintainedat a value or values not in excess of the minimum thickness of the webin the forming zone. Consequently, there is no opportunity for thefibrous web to expand, thus resulting in displacement of the fiberswhich might affect efiicient washing, in the washing Z0116. A washingliquid (usually water) is preferably supplied to the washing zone underpressure, a housing being associated with the liquid pervious member todefine a washing liquid supply chamber outwardly of the washing zone andhaving one or more inlets. A press roll adjacent the downstream end ofthe washing zone urges the wire and web against the drum surface toexpress wash liquid from the web into the drum, and the suspensionliquid expressed from the fibrous material in the web-forming zone anddisplaced from the web in the washing zone, as well as the washingliquid removed by the press are withdrawn from within the drum. Afterpressing, the web is removed and disinte- 7 of the web. Moreover, theconfinement of the fibrous material between the drum surface and wire,which is backed up by the member, allow the application of relativelyhigh pressure to the web near the downstream end of the web-forming zoneso that the web produced in the web-forming zone has a relatively highdry content. Consequently, the amount of washing liquid required todisplace the remanent liquid is minimized in the apparatus.

With the apparatus, the wash liquid may be supplied under a relativelyhigh pressure, thereby providing effective displacement of the liquidsfrom the web and into the drum. Moreover, because the Web is notpermitted to expand and is carried between conjointly moving surfaces,the opportunity for displacement and relative movements of the fibers issubstantially eliminated, thereby enhancing the effectiveness of thewashing.

The washing zone may be divided into two or more circumferentialsections. Clean washing liquid is supplied to the downstream section,and the liquid displaced from the web in the downstream washing sectionis separately collected within the drum and is supplied to the nextupstream washing section. For example, if there are three sections,clean washing liquid is supplied to the downstream section, and theliquid displaced into the drum is collected and supplied as a washingliquid for the intermediate section. The liquid displaced into the drumin the intermediate section is collected within the drum and isdelivered as the wash liquid to the upstream section. This arrangementis particularly advantageous from the standpoint of minimum washingliquid requirements.

The member defining the web-forming zone is preferably formed andpositioned, relative to the drum surface, so that the thickness of theWeb-forming zone decreases linearly in the direction of rotation of thedrum. Preferably, the web-forming zone is designed to provide a webdryness at the end of the zone of at least 18 percent, and preferably onthe order of 33 percent. A dryness of approximately 18 percent meansthat substantially all liquid in the void spaces between the fibers andalso a part of the lumen liquid, is removed. At a solids content of..33vpercent all lumen liquid has been removed. In the washing zone, thedryness of the web will remain at substantially the same or even ahigher value than at the end of the web-forming zone, inasmuch as theweb is constrained by the liquid-pervious member defining the washingzone and is prevented from expanding. In general, the washing efficiencyis a function of the ratio of the volume of washing liquid to the volumeof suspension liquid retrained in the web. Thus, the higher the solidcontent in the web formed in the forming zone, the less the washingliquid to obtain a given washing efliciency. Similarly, the apparatusmakes it possible to increase washing efiiciency significantly with agiven amount of washing liquid.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention,reference may be made to the following description of exemplaryembodiments, taken in conjunction with the figures of the accompanyingdrawing, in which:

FIG. 1 is an end sectional view in generally schematic form, showing oneembodiment of the invention; and

FIG. 2 is an end view, also generally schematic, of another embodimentof the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Except to the extent that will bedescribed below, the two embodiments are the same, and therefore thesame reference numerals are applied to the corresponding parts of each,and the following description may be read in conjunction with both ofthe figures.

The embodiments of the apparatus shown in the drawing comprise a drum 3,which is mounted for rotation about a shaft 1 and is provided with aperforated shell 5, such as a shell provided by a perforated bodycovered with a foraminous wire. The shell is, of course, permeable toliquids but is substantially impermeable to the fibrous material to betreated. A foraminous belt 7, such as a wire mesh, is looped around themajor portion of the drum surface, and is arranged to move conjointlywith the drum surface in spaced relation. Beginning at the point nearwhere the belt 7 is led to the drum surface and extending to over aportion of the drum surface in spaced relation is a first rigid member,such as a curved plate 9, that defines with the drum surface aweb-forming zone 13. The plate may be non-pervious to liquid butpreferably is liquid-pervious, for example, by virtue of perforations.The plate 9 is formed and positioned relative to the drum surface todefine a convergent web-forming zone 13 that diminishes in thickness inthe direction of rotation of the drum surface, preferably as a linearfunction of the distance measured along the drum surface. A rigidliquidpervious member 11, which may also be a perforated plate, extendsfrom a point adjacent the end of the plate 9 over another portion of thedrum surface, in particular the major portion of the remaining part ofthe drum encompassed by the wire 7, and is formed and positionedrelative to the drum surface to define a washing zone 15. In spacedrelation outwardly of the plate 9 is a collector or casing 19 having anoutlet 21, and associated with the plate 11 is a housing 25 having oneor more inlet couplings 27 for introducing wash liquid into the washliquid supply chamber. The plates 9 and 11 are mounted on the casings l9and 25, and the casings are preferably connected together by a pin 29.The casings are carried by adjustable devices 31 that provide forpositioning the liquid-pervious members 9 and 11 in desiredpredetermined positions relative to the drum. Advantageously, thedevices 31 are mechanisms that are responsive to pressures acting on themembers 9 and 11, such as hydraulic cylinders, and they are arranged toprovide controllable pressures for particular operating conditions ofthe apparatus.

As will be described below, the foraminous belt 7 is arranged to travelthrough the apparatus in engagement with the members 9 and 11 and is ledaway from the drum around a press roll 37 located downstream from theend of the washing zone. The wire then leads to a carrier roll 39, turnsaround a. tension roll 41 and is led back to the drum by another carrierroll 39'.

The suspension of fibrous material to be washed is introduced betweenthe wire 7 and the drum surface through a distribution box 43 locatedwithin the run of the wire from the press roll 37 back to the roll 39,the distribution box having a slice that provides delivery of thefibrous stock uniformly across the length of the drum. Flexible seals 45are located at the inlet from the slice to seal the space between thedrum surface and the wire. Also within the run of the wire 7 between thepress roll 37 and the.

roll 39 is a conveyor screw 47 for removing the web from the apparatus.The ends of the drum are closed by end walls appropriately sealed to thedrum and the members 9 and 11 to confine the web-forming zone andwashing zone, and drains 51 are provided through the end walls to removeliquids from within the drum.

The embodiments further include apparatus for controlling the thicknessof the fibrous web, such apparatus including an instrument 53, which maybe any suitable mechanical, electro-mechanical or electronic device fordetecting changes in the position of the member 11 in response to anychange in thickness of the web. The instrument 53 is connected (asrepresented by the line 55) to a controller 57, and the controlleroperates a servovalve 59 of an appropriate type in the supply conduit.Many types of control systems are well known to those skilled in theart, and therefore the details of a particular system need not bedescribed or shown herein. Suffice it to say that a change in thicknessof the web owing to some variation in the operating conditions willproduce an adjustment in the supply of stock appropriate to return theweb thickness to a desired value. As an alternative to the control ofthe supply of fibrous suspension to the apparatus, as in the embodiment,it would also be appropriate to monitor the thickness of the web andcontrol the rate of rotation of the drum in accordance with thethickness. If the thickness of the web increases over a desired value,then the speed of operation of the apparatus will be increased, andvice-versa.

The apparatus of FIG. 1 operates in the following manner. The fibroussuspension is supplied to the distribution box 43 at a predeterminedrate, as described above, and it is evenly distributed across the inletend of the webforming zone 13. The suspension is delivered underpressure so that it is forced into the converging space between the wire7 and the surface of the drum 5. The wire moves conjointly with the drumsurface due to friction between the drum surface and fibrous webs andthe web and wire. As the fibrous material sandwiched between the drumsurface and wire moves along the forming zone, it is drained underpositive pressure outwardly through the wire and the plate 9. Liquidexpressed outwardly is collected by the casing 19 and is removed throughthe outlet 21. At the same time, the suspension is also drained inwardlyinto the drum through the drum shell, and the liquid is removed throughthe outlets 51. As the fibrous material progressively drains, it isformed into a web between the drum shell and the wire 7. The pressuregenerated in the fibrous material along the forming zone is sufficientto cause the web to leave the forming zone with a relatively high solidscontent, say at least 18 percent and preferably on the order of 33percent.

The web of fibrous material is then conducted from the web-forming zone13 to the washing zone where it comes under the influence of thepressure of the plate 11 which, as described above, is positioned tomaintain a web thickness not in excess of the minimum thickness achievedin the web-forming zone. Washing liquid is supplied under pressurethrough the inlet 27 to the washing liquid supply chamber formed by thecasing and is forced into the web, thereby displacing suspension liquidthen entrained in the web and causing the suspension liquid to drainthrough the shell of the drum into the interior for removal through theoutlets 51. Desirably, the devices 31 maintain the pressure throughoutthe washing zone at a substantially constant value, so that the drycontent of the web in the washing zone remains substantially constant.The web, still sandwiched between the wire and drum shell, then passesthrough the nip formed by the press roll 37 and drum is dewatered, sayto a dryness of on the order of 50 percent. Although the press roll 37may be a solid roll, it is advantageous to use a perforated suctionpress roll 37 so that dewatering of the web takes place not onlyinwardly through the drum shell but outwardly through the wire 7 andinto the press roll 37. The web is then delivered to the conveyor screwand is removed by the conveyor screw from the apparatus, the process ofremoval also resulting in breaking up the web and discharging thefibrous material in bulk form for further processing.

The apparatus of FIG. 2, as mentioned above, is substantially the sameas the embodiment of FIG. 1. However, the embodiment of FIG. 2 has amodified washing system. In particular, the washing zone 15 is dividedinto three sections 63, 63 and 63", each washing section having aseparate casing defining a separate wash liquid supply chamber andprovided with a separate connection 27, 65, and 67, respectively.Located within the drum inwardly of the sections 63 and 63 are separatecollectors 69 and 71, each of which has an outlet 73 and 75,respectively. The collector 69 serves the intermediate washing section63', and the collector 71 serves the downstream section 63" as well asoverlapping the press roll 37 so that liquid expressed from the web bythe press roll 37 is collected in it.

In the embodiment of FIG. 2, clean wash liquid is supplied to thedownstream wash section 63" through the inlet 67 and washes the web inthe manner described above in connection with the embodiment of FIG. 1by displacing liquid then entrained in the web and forcing the entrainedliquid through the drum shell into the collector 71. The liquiddisplaced into the collector 71, together with the liquid expressed fromthe web by the press roll 37, is collected and withdrawn through theoutlet and is sup plied under pressure to the inlet 65 of theintermediate wash section 63. This wash liquid washes the web, in themanner described above, and the liquid collected in the collector 69 iswithdrawn to the outlet 73 and is supplied under pressure to the inlet27 to the first or upstream wash section 63, again washing the web anddisplacing liquid from the web through the shell and into the interiorof the drum where it is removed through the main outlet 51.

Although the embodiments of FIGS. 1 and 2 are provided withliquid-pervious members 9 defining the webforming zone, a solid membermay be substituted, thus providing drainage of the web only inwardlythrough the shell. Also, the embodiment of FIG. 2 may be furthermodified by omitting one of the three units or sections of the washingzone or by adding additional sections. Thus, the embodiments of theinvention described above and illustrated in the drawings are intendedto be merely exemplary, and those skilled in the art will be able tomake numerous variations and modifications of them without departingfrom the spirit and scope of the invention.

I claim:

1. Apparatus for washing a fibrous material suspended in a liquidcomprising a drum having a shell pervious to the liquid andsubstantially non-pervious to the fibrous material; means for rotatingthe drum, means including a rigid curved plate-like member positioned toencompass a substantial segment of the drum shell and defining a webforming zone along a portion of the periphery of the drum shell, theforming zone being a space bet-ween the drum and member and beingtapered in the direction of rotation of the drum, means including arigid curved liquid-pervious plate-like member positioned to encompass asubstantial segment of the drum surface and defining along a secondportion of the drum surface downstream from the first portion, relativeto the direction of rotation of the drum, a washing zone, a flexibleforaminous belt trained around the drum within the rigid member and theliquid-pervious member and mounted for movement with the drum surface,means for introducing the suspension of fibrous material in a liquid atthe upstream end, relative to the direction of rotation of the drum, ofthe web-forming zone between the belt and the drum shell, wherebysuspension liquid is drained from the fibrous material through the drumshell and a web of the fibrous material formed between the drum shelland the belt, means associated with the rigid member for maintaining itat a spacing from the drum surface to define the web-forming zone and toresist outward pressure of the belt as the fibrous suspension is drainedof liquid under pressure to form the web, means associated with theliquid-per'vious member for maintaining it at a spacing from the drumsurface not greater at any point therealong than the minimum dimensionof the web-forming space so that the web is not permitted to expand inthe washing zone, means for supplying a washing liquid under pressure tothe washing zone from outside the liquid-pervious member to displaceliquid in the fibrous web from the web into the interior of the drumthrough the shell, means communieating with the interior of the shellfor withdrawing therefrom liquids expressed and displaced from thefibrous material in the web-forming and washing zones and meansdownstream from the washing zone and upstream from the web-forming zonefor removing the web from the apparatus.

2. Apparatus according to claim ll wherein the webforming zone andwashing zone are contiguous at the downstream and upstream ends thereof,respectively, and wherein the rigid members are pivotally connectedtogether adjacent the juncture between such respective downstream andupstream ends.

3. Apparatus according to claim 1 further comprising a press meansdownstream from the wash zone for urging the belt and web against thedrum surface to express washing liquid from the web prior to its removalfrom the apparatus.

4. Apparatus according to claim 1 wherein the means for introducingwashing liquid to the washing zone comprises a housing in spacedrelation to the said liquidpervious member defining the Washing zone,the housing defining a wash liquid supply chamber and having at leastone inlet for receiving washing liquid.

5. Aparatus according to claim 11 wherein the means for introducing thesuspension liquid to the web-forming zone is a slice adapted to supplythe suspension liquid uni formly across the web-forming zonelongitudinally of the drum.

6. Apparatus according to claim 1 wherein the rigid member defining theWeb-forming zone converges uniformly towards the drum surface in thedirection of rotation of the drum.

7. Apparatus according to claim 1 wherein the rigid member defining theweb-forming zone is pervious to liquids.

8. Apparatus according to claim 7 and further comprising means defininga housing in space relation to and outwardly of the said rigid memberfor receiving and collecting suspension liquid expressed from thefibrous material in the Web-forming zone, and means associated with thehousing for conducting away the suspension liquid collected therein.

'9. Apparatus according to claim 1 wherein the washing zone comprises atleast two circumferential sections including an upstream section and adownstream section and wherein the means for supplying washing liquid tothe washing zone includes a first supply means for supplying wash liquidto the upstream section and second supply means for supplying washliquid to the downstream section.

10. Apparatus according to claim 9 wherein the first and second supplymeans include housing members in spaced relation to the rigidliquid-pervious member and defining, respectively, first and secondsupply chambers outwardly of the respective first and second sections ofthe washing zone and at least one inlet communicating with each of thesaid first and second supply chambers for supply of washing liquidthereto' 11. Apparatus according to claim lit further comprisingcollector means within the drum inwardly of the downstream. washing zonesection for collecting liquids expressed and displaced from the web inthe said downstream washing zone section, means coupled to saidcollector for removing liquids therefrom, and wherein the first supplymeans receives liquid from the said collector means.

12. Apparatus according to claim 1 wherein the washing zone includesthree contiguous circumferential sections, and further comprisingcasings associated with the rigid liquid-pervious member definingindividual washing liquid supply chambers associated with each of saidthree sections, at least one wash inlet associated with each supplychamber, a collector located inwardly of each of the downstream two ofthe three sections for receiving liquids expressed and displaced fromthe web in each such two sections, means associated with the lastdownstream collector for removing liquids therefrom and introducing themto the intermediate section, and means associated with the collector forthe intermediate section for removing liquids therefrom and introducingsuch liquids to the upstream section.

13. Apparatus according to claim 12 further comprising a press rolldownstream of the downstream wash zone section for urging the wire andweb against the drum surface to express liquid from the web, and whereinthe downstream collector is positioned to receive liquids expressed fromthe web by the pres roll.

14. Apparatus according to claim 1 further compris ing means formonitoring the thickness of the web and producing a signal indicativethereof, and means responsive to the signal for controlling thethickness of the web.

15. Apparatus according to claim 14 wherein the means for controllingthe thickness of the Web comprising means responsive to the said controlsignal for varying the rate of delivery of suspension material to theweb-forming zone.

References Cited UNITED STATES PATENTS 3,025,69l 3/1962 Fleissner 68l583,236,724 2/1966 Wahlstrom 162 -3l7 3,270,532 9/1966 Chaikin et al 68453,397,112 8/1968 Highton et al. 162-318 JOSEPH H. MCGLYNN, PrimaryExaminer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3 616 660 Dated November 2 1971 Inventods) Gosta Ingemar IngemarssIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

On the drawings sheets 1 and 2, "6.1. INGERMARSSON", each occurrence,should read G. I. INGEMARSSON In the heading to the printedspecification, line 4, Costa Ingemar Ingermarsson" should read GostaIngemar Ingemarsson Column 8, line 48, "pres" should read press line 55,"comprising" should read comprises Signed and sealed this 9th day of May1972.

[SEAL] Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents M PO'1OSO (1069) USCOMM-DC 60376-P69 U.$, GOVERNMENT FHINHNGOFFICE iil 0-366334

